110 PLATYHELMINTHES [CH. 



invagination of part of the outer layer now takes place, forming 

 a pouch, on the inner side of which the suckers and hooks of the 

 adult head make their appearance. The pouch is then turned 

 inside out and a well-marked head is thus formed. The larva is 

 now known as a bladder-worm or Cysticercoid and is found 

 parasitic in a great number of animals. A very common form is 

 Cysticercus pisiformis found in the coelom of the Rabbit attached 

 to the mesentery (see p. 437). Coenurus cerebralis is a dangerous 

 Cysticercoid in which the bladder can become as large as a plum 

 and on which not one but numerous heads are formed. It is 

 found lodged in the brain of the Sheep and other domestic animals 

 and causes the disease known as gid or staggers. A still more 

 dangerous form is Echinococcus polymwphus, in which the bladder 

 may become as large as & man's head. This enormous vesicle buds 

 off from its inner surface secondary vesicles on each of which 

 numerous heads are formed. This parasite is found in the Pig, 

 Sheep, and even Man, in the liver and other internal organs. 



The Cysticercoid can complete its development only when 

 its first host is eaten by another animal. Then the bladder is 

 cast off whilst the head firmly attaches itself by its hooks and 

 suckers to the alimentary canal of its new host and commences to 

 bud off proglottides. Though the adult may attain an immense 

 length (as much as 20 feet) it is a much less dangerous parasite 

 than the larva, and rarely produces worse symptoms than giddiness 

 and a certain amount of abdominal pain. The adults are found 

 chiefly in carnivorous animals, above all in the Dog, "Wolf and allied 

 species. Thus Cysticercus pisiformis becomes Taenia serrata, 

 Coenurus cerebralis gives rise to Taenia coenurus, and ^Echinococcus 

 polymorphus to Taenia echinococcus, all three infesting the ali- 

 mentary canal of the Dog and its allies. The common Taenia 

 solium found in the human intestine is developed from a Cysticercoid 

 found in the muscles of the Pig. 



Bothriocephalus latus, which may attain a length of 30 feet, is 

 the largest Cestode found in Man. It belongs to a more primitive 

 division of the class than Taenia, for it gives rise to a free- 

 swimming ciliated larva, described above, which is swallowed by the 

 Pike or the Perch. In this it develops into a solid larva termed 

 a Plerocercoid which gives rise to the adult when the fish is 

 eaten by Man. 



It is obvious that an animal which like a Digenetic Trematode 

 or a Cestode depends for its survival on such a combination of 



